The present invention relates to an ink-jet printing method and apparatus which print an image based on image information on a printing medium by moving relatively each ink-jet printhead and the printing medium. Note that the present invention can be applied to copying machines, facsimile apparatuses having communication systems, wordprocessors and the like having printing units, and industrial printing apparatuses combined with various processing apparatuses as well as general printing apparatuses.
In an ink-jet printing apparatus having a serial type of ink-jet head, the ink-jet head is driven to discharge ink from each nozzle in synchronism with the scanning operation of the ink-jet head on the basis of a head driving signal to scan the ink-jet head in the main scanning direction, thereby forming an image on a printing medium such as a printing sheet.
A printhead used for an ink-jet printing apparatus will be described next.
Ink-jet printing schemes typically include a scheme using an electrothermal transducer (heater) as an element for generating discharge energy to eject ink droplets or a scheme using a piezoelectric element. In both schemes, ink is ejected by supplying an electrical signal to an element for generating discharge energy. An advantage of the former scheme is that only a small space is required to place each heater serving as an element for generating discharge energy. This makes it possible to simplify the arrangement of an ink-jet printhead and hence reduce its size. In addition, it is relatively easy to achieve an increase in density. A disadvantage of this scheme is that the heat generated by each heater is accumulated in the printhead. As a consequence, ejected ink droplets tend to vary in volume. In addition, shock (cavitation) produced when air bubbles generated to eject ink shrink and disappear greatly affect the heaters.
As methods of solving these problems, for example, ink-jet printing methods are disclosed in Japanese Patent Laid-Open Nos. 54-161935, 61-185455, 61-249768, and 4-10941. According to these methods, each ink-jet head has an orifice for discharging a liquid, an ink channel which communicates with the orifice and is filled with ink, and an electrothermal transducer placed in the ink channel. This electrothermal transducer is generally formed by a thin resistive element. According to a characteristic feature of this electrothermal transducer, a pulse-like current is supplied (application of a driving pulse) to the electrothermal transducer through an interconnection to generate heat energy. By using such a printing method, the stability of the volume of each ink droplet can be improved, and small droplets can be ejected at high speed. This makes it possible to improve the durability of each heater by solving the problem of cavitations produced by printing and defoaming.
When, however, images are continuously printed by using such a conventional ink-jet head with a high duty, the temperature of the head rises. With this rise in temperature, the bubble size increases due to the driving operation for ejecting ink droplets, and the ink refill operation becomes insufficient for the next ink ejecting operation, resulting in a deterioration in the driving frequency characteristics of the head. If the printhead is driven to eject ink before the printhead is sufficiently refilled with ink, ejected ink droplets vary in amount. As a consequence, ink droplets land disorderly, and hence the image quality greatly deteriorates.
If the driving frequency of each head in printing operation using an ink-jet printing apparatus is uniformly set to be low in consideration of a deterioration in the driving frequency characteristics of the printhead which is caused by a rise in the temperature of the head, the overall throughput decreases.
A method of detecting the temperature of each printhead and controlling the driving frequency of the printhead in accordance with the detected temperature is disclosed in Japanese Patent Laid-Open No. 1-308647. In this case, the scanning speed of the printhead is decreased and the driving frequency is decreased immediately after the temperature of the printhead exceeds a predetermined temperature. For this reason, when the scanning speed of the printhead has been changed, a deterioration in image, e.g., density unevenness and streaks, occurs.
An ink-jet printing apparatus and a control method for the apparatus which switches the head driving frequencies in accordance with the environmental temperature are disclosed in Japanese Patent Laid-Open No. 60-210480. This head driving frequency switching is performed on the basis of the environmental temperature obtained by a temperature detector mounted outside each printhead. The internal temperature of each ink-jet head locally rises greatly when, for example, the head is continuously driven to eject ink. It is therefore difficult to satisfactorily cope with a change in temperature by head driving control based on only the environmental temperature outside the printhead. In addition, in general, the environmental temperature does not abruptly change within a short period of time. However, the head temperature abruptly changes when several pages are continuously printed. Therefore, the timing at which a head temperature is detected and the timing at which the driving frequency is controlled are important. According to Japanese Patent Laid-Open No. 60-210480, the driving frequencies are controlled on the basis of only the environmental temperature. With regard to this control, therefore, this reference discloses only a technique of simply switching driving frequencies of the printhead depending on whether the environmental temperature is higher or lower than a reference temperature.
The present invention has been made in consideration of the prior arts described above, and has as its object to provide an ink-jet printing method and apparatus which can prevent a deterioration in image due to a deterioration in ink refill characteristics with a rise in temperature inside an ink-jet printhead and minimize a decrease in printing speed.
It is another object of the present invention to provide an ink-jet printing method and apparatus which can prevent a deterioration in image by suppressing a rise in the temperature of an ink-jet printhead.
In order to attain the objects, the present invention of an ink-jet printing apparatus comprises the structure as follows.
An ink-jet printing apparatus for printing an image based on image information on a printing medium by relatively moving an ink-jet printhead and the printing medium, comprises: temperature detection means for detecting a temperature of the ink-jet printhead and outputting temperature information; acquisition means for acquiring the temperature information detected by said temperature detection means; and control means for performing control to switch driving frequencies of the ink-jet printhead in units in which the image information does not continue in accordance with a comparison result on the temperature information acquired by said acquisition means and a predetermined temperature.
In order to attain the objects, the present invention of an ink-jet printing method comprises the steps as follows.
An ink-jet printing method for an ink-jet printing apparatus for printing an image based on image information on a printing medium by relatively moving an ink-jet printhead and the printing medium, comprising the steps of: acquiring a temperature of the ink-jet printhead; and controlling to switch driving frequencies of the ink-jet printhead in units in which the image information does not continue in accordance with a comparison result on the temperature information acquired in said acquiring step and a predetermined temperature.
Other features and advantages of the present invention will be apparent from the following description taken in conjunction with the accompanying drawings, in which like reference characters designate the same or similar parts throughout the figures thereof.